1. Technical Field
The present invention relates to switching converters and to specifically a method and devices for zero current switching for reducing switching losses in switching converters.
2. Description of Related Art
FIG. 1 shows a typical conventional buck-boost DC-to-DC converter circuit 10. The buck circuit of buck-boost DC-to-DC converter 10 has an input voltage Vin with an input capacitor C1 connected in parallel across Vin. Two switches are implemented as field effect transistors (FET) with integral diodes: a high side buck switch Q1 and a low side buck switch Q2 connected in series by connecting the source of Q1 to the drain of Q2. The drain of Q1 and the source of Q2 are connected parallel across an input capacitor C1. A node A is formed between switches Q1 and Q2 to which one end of an inductor 106 is connected. The other end of inductor 106 is connected to the boost circuit of buck-boost DC-to-DC converter 10 at a node B. Node B connects two switches: a high side boost switch Q4 and a low side boost switch Q3 together in series where the source of Q4 connects to the drain of Q3 to form node B. The drain of Q4 and the source of Q3 connect across an output capacitor C2 to produce the output voltage Vout of buck-boost DC-to-DC converter 10.
FIG. 2a illustrates the buck phase or on-state circuit of DC-to-DC converter circuit 10 shown in FIG. 1, the input voltage source Vin is directly connected to inductor 106 and the load is isolated from Vin because Q1 is on, Q2 is off, Q3 is on and Q4 is off. These switch positions: Q1 on, Q2 off, Q3 on and Q4 off; result in accumulating energy in inductor 106 since source Vin is directly connected to inductor 106. In the on-state, output capacitor C2 supplies energy to the load.
FIG. 2b illustrates the boost phase or off-state circuit of DC-to-DC converter circuit 10, Inductor 106 is connected in parallel across the load and capacitor C2 because Q1 is off, Q2 is on, Q3 is off and Q4 is on. Q1 being off isolates inductor 106 from the input voltage (Vin) and capacitor (C1). The stored energy in inductor 106 (as a result of the previous On-state) is transferred from inductor 106 to C2 and the load.
Two common methods of operating DC-to-DC converter circuit 10 are in either continuous mode or discontinuous mode. If the current through the inductor 106 never falls to zero during a commutation cycle (i.e. the time period to perform both the on-state and the off-state), DC-to-DC converter circuit 10 is said to operate in continuous mode and typically the on-state operates for a shorter period of time when compared to the off-state. Discontinuous mode of operation for DC to DC converter circuit 10 occurs when the amount of energy required by the load is small enough to be transferred in a time period smaller than the whole commutation cycle. Typically, the current through inductor 106 falls to zero for a short time period after the off-state period and therefore inductor 106 is completely discharged at the end of the commutation cycle. The commutation cycle therefore includes the on-state, the off-state and the short time period during which the inductor current is zero.
A conventional “resonant” method for achieving virtually zero power loss when switching a switch is to apply a direct current voltage input voltage Vin across a switch (with a diode connected across the switch, the diode is reverse biased with respect to Vin) in series with an inductor L and a capacitor C. The output voltage of the circuit is derived across the capacitor. The output voltage of the circuit could then in principle be connected to the input of a power converter, for example a buck-loaded series tank circuit with load. The resonant frequency of the series inductor L and capacitor C is given by Eq. 1 and the corresponding resonant periodic time T given in Eq. 2.f0=½π(LC)1/2  Eq.1T=1/f0  Eq.2
A pulse response of the circuit means that when the switch turns on, there is both zero current in the inductor and zero voltage across the capacitor (Power=Volts×Current=0×0=zero power loss at turn on). During steady state operation of the circuit, the inductor current and capacitor voltage are sinusoidal and have a 90 degrees phase shift with respect to each other. When the switch turns off (the on period of the switch corresponds to half of the resonant periodic time) there is zero current in the inductor and maximum positive voltage (i.e. Vcapacitor=Vin) across the capacitor (Power=Volts×Current=Vin×0=zero power loss at turn off).